1. Field of the Invention
This invention relates to treatment of toxic wastes, and more particularly, to treatment of such wastes containing heavy metals.
2. General Discussion of the Background
A heavy metal is a metal having a specific gravity greater than about 5.0. Examples of such metals are aluminum, cadmium, lead, cobalt, copper, thallium, zinc, antimony, chromium, nickel, and mercury. Arsenic and selenium are also classified as heavy metals by environmental regulatory agencies, although they are chemically classified as nonmetals. As used herein, the term heavy metals includes arsenic and selenium.
It is believed that almost 60% of the toxic wastes generated commercially in the United States each year are derived from primary metal or electroplating industries, which produce heavy metals. A major survey of four northeastern states has helped confirm this estimate by determining that heavy metal wastes constitute 41% of all hazardous wastes produced annually in those states. Greenberg & Anderson, "Hazardous Waste Sites: the Credibility Gap," The Center for Urban Policy Research, New Brunswick, New Jersey (1984), page 19.
Although heavy metals are a serious environmental contaminant, no economically feasible process exists for detoxifying them. Heavy metal wastes are typically immobilized by enclosing them in a container or altering their form, sometimes through solidification of the waste. Typical solidification processes include forming the waste into a ceramic or resin mixture, mixing the waste with industrial by-products such as dust or shale, or solidifying them with cement. Hazardous Wastes Management Book, Butterworth & Company, London (1985), page 149.
Typical cement based processes encapsulate waste in a solid matrix to physically encapsulate the waste without altering it chemically. Such cement based processes form an environment with a pH of about 9-10, which is a region of minimum solubility for metal hydroxides produced by encapsulation of the heavy metals within cement. The soluble metal hydroxides leach out of the cement matrix as the matrix is deteriorated by exposure to the environment. Cement encapsulation of wastes is therefore unsuitable for permanent disposal of toxic materials such as heavy metals.
Another waste product treatment process is disclosed in U.S. Pat. No. 3,837,872 which was reexamined and subsequently issued as reexamination Patent B1 3,837,872. The U.S. Pat. No. 3,837,872 process concerns treatment of sewage sludge and other wastes by chemical fixation and physical entrapment of pollutants within an alkaline silicate matrix. The waste is treated by mixing it with a setting agent and silicate, preferably in a waste to setting agent to silicate ratio of about 10:1:0.5 by volume. The resultant product is a friable, clay-like mass having a polymer lattice that entraps and reduces migration of toxic materials such as some metals and organics.
The U.S. Pat. No. 3,837,872 process is superior to mere physical encapsulation in a cement matrix because the silicate reactions of the U.S. Pat. No. 3,837,872 process chemically alter the waste in addition to physically encapsulating it within an alkaline silicate matrix. Unfortunately, many anionic and nonionic forms of metals are not retained within the alkaline silicate matrix. The silicate anions in the matrix are double trigonal and tetragonal rings of (Si.sub.6 O.sub.15).sup.-6, (Si.sub.8 O.sub.20).sup.-8, and (Si.sub.8 O.sub.18 (OH).sub.2).sup.-6. These silicate anions have an affinity for cations and do not chemically and physically entrap Pearson soft metals in the alkaline environment of the cement-silicate matrix formed by the treatment process.
Another common heavy metal pollutant is arsenic, which is produced in the flue dust of copper and lead smelters. Arsenic is highly toxic when ingested or inhaled and is a known carcinogen. In spite of these drawbacks, arsenic is widely used in manufacturing metals, semiconductors, and some medications. Other forms of arsenic include arsenate (AsO.sub.4).sup.-3 and arsenite (AsO.sub.3).sup.-3. Although elemental arsenic is immobilized in the cement-silicate matrix formed by the U.S. Pat. No. 3,837,872 process, the highly negatively charged arsenate and arsenite easily migrate in the cement-silicate matrix.
Another industrial pollutant is selenium. It is used in Xerographic plates, photocells, magnetic computer cores, solar batteries, ceramics, and other products. The cement-silicate matrix of the U.S. Pat. No. 3,837,872 process does not immobilize selenium.
It is accordingly an object of the present invention to provide a process for physically and chemically encapsulating heavy metals which are not already bound by an alkaline silicate matrix.
Another object is to provide a process which chemically alters and physically encapsulates heavy metals.
Yet another object is to provide an economically feasible process for rendering wastes nonpolluting, and which satisfies government regulations for the disposal of such wastes.
Even yet another object is to provide a method of waste treatment of heavy metals and arsenic which results in an environmentally nonhazardous product which can be used for economically beneficial purposes such as landfill.
Finally, it is an object of this invention to provide such a treatment process which can be performed on a continuous basis to treat large volumes of toxic waste containing heavy metals and arsenic.